Bald Eagle - mature color phase
|Prior to the 1985, bald eagles were not seen along the Colorado River in the Grand Canyon. Biologists performing bird surveys in the Grand Canyon during the 1970's and early 1980's recorded no bald eagles in the river corridor. The eagles began to winter in the canyon because they discovered new food sources. Rainbow trout and wintering waterfowl increased as a result of the completion of Glen Canyon dam in 1963. Before the completion of Glen Canyon dam, the Colorado River was muddy. Water releases from Glen Canyon dam are clear because sediments drop out in the still water behind the dam. This phenomenon is not new to the Colorado river, dams created by lava flows created similar conditions many times in the past. The clear water in the river now allows sunlight to reach the river bottom, causing a green algae, Cladophora glomerata, to flourish. Cladophora provides food and shelter for aquatic insects and crustaceans that are eaten by the trout. The increased aquatic primary productivity in the river also supports wintering waterfowl that are another important food source for the eagles. Bald eagles have learned to exploit these new resources. Lake Powell is also an important habitat for wintering bald eagles. Surveys have found 30 to 45 bald eagles wintering around Lake Powell since the early 1980's. Concentration of eagles in this region follows changes in availability of prey. When large numbers of trout from the Colorado river spawn in Nankoweap Creek, the eagles congregate there. The majority of raptors don't live to adulthood due to chance environmental factors effecting availability of prey during the critical learning period. An unusually high ratio of juvenile to adult bald eagles assemble at Nankoweap when spawning trout are abundant, suggesting that this resource may be important to the future of the bald eagles in the southwest.|
Immature Bald Eagle
Carothers, S.W. and B.T. Brown. 1991. The Colorado River through Grand Canyon. University of Arizona Press, Tucson
Carothers, S.W. and D.A. House. Birds of prey on the Colorado Plateau, Plateau, vol. 63, no.3, pp. 32-35, Museum of Northern Arizona.
Hamblin, W.K. and L. Hamblin. 1997. Fire and Water, The Making of the Grand Canyon. Natural History. vol. 106, no.8, pp.34-40
van Riper III, C., Sogge,M., Tibbitts,
T., Wintering Bald Eagles Along the Colorado River Corridor.
Our Living Resources. A Report to the Nation on the Distribution,
Abundance, and Health of U.S. Plants, Animals, and Ecosystems
U.S. Dept of the Interior, National Biological Service. U.S Government Printing Office, Washington D.C.
U.S. Department of the Interior. 1995. Final Environmental Impact Statement, Glen Canyon Dam
Zwinger, A. H., 1995. Downcanyon, A Naturalist Explores the Colorado River through the Grand Canyon. The University of Arizona Press
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